1. Field of the Invention
The present invention relates to a wavelength allocation method using wavelength division multiplexing (WDM) technology, a transmission equipment and receiving equipment using this method, and a wavelength multiplex transmission system, and in particular, a wavelength allocation method which carries out wavelength multiplexing by allocating the carrier frequency (hereinbelow, referred to as xe2x80x9cfrequencyxe2x80x9d) of the signal beam of each channel with unequal spacing in order to mitigate the influence of nonlinear effects of the optical fiber, a transmission equipment and a receiving equipment using this method, and a WDM transmission system.
2. Background Art
In a 1.55 xcexcm dispersion shifted fiber, the zero-dispersion wavelength is set in the region of 1.55 xcexcm, which is around the signal frequency, so that waveform distortion is not produced by fiber chromatic dispersion, and thereby, this fiber can also be applied on high speed transmission.
However, when shifted fiber is employed to WDM systems, because the wavelength dispersion in the signal wavelength is small, there is the problem that a new optical power that is newly generated by four wave mixing, which is one of the nonlinear effects of an optical fiber, becomes large. When this newly generated four wave mixing light has the same frequency as the signal frequency, this four wave mixing light becomes noise, and transmission distance is restricted. In order to relieve this problem, a method of unequally allocating the frequency intervals of the signal frequency has been proposed.
Here, let the frequencies of the four wave mixing light generated from three signal light with frequencies fi, fj, and fk (kxe2x89xa0i, j) be expressed by fijk. Next equation is satisfied,
fijk=fi+fjxe2x88x92fkxe2x80x83xe2x80x83(1)
FIGS. 6A and 6B shows the position where the four wave mixing light is generated. In FIGS. 6A and 6B, the frequencies of the signal light in Channels 1, 2, and 3 are f1, f2, and f3, respectively. 12 four wave mixing lights are generated.
FIG. 6A shows the four wave mixing light when the frequencies of each signal beam are allocated with equal spacing. For example, f223 denotes the frequencies of the four wave mixing light when fi=fj=f2, and fk=f3, and shows that the four wave mixing light generated by the signal beams of channels 2 and 3 overlaps the signal beam of channel 1. This is identical for f132, f312, and f221. Because this kind of four wave mixing light cannot be separated from the signal light using, for example, an optical filter, it generates cross-talk in the signal beam, and the S/N ratio is degraded.
FIG. 6B shows the four wave mixing light when the frequencies of each signal beam are unequally spaced. As shown here, by unequally spacing the frequencies of each signal, the each signal frequency and the frequencies of the four wave mixing light do not coincide, it becomes possible to separate with optical filters, and the influence of the four wave mixing light can be decreased. Moreover, in the present specification, the frequency interval of any two arbitrary channels is different from the frequency intervals of all other pairs of channels, and the frequency interval of signal beams being set so that the frequencies of the signal beams and the four wave mixing light do not coincide at all is called xe2x80x9ccompletely unequally spaced channel allocationxe2x80x9d.
An algorithm that determines the frequency interval based on this type of completely unequally spaced channel allocation is disclosed in Japanese Patent Application, First Publication, No. Hei 7-264166 (Japanese Patent Application, No. Hei 7-29043; xe2x80x9cMultiplexed Channel Optical Fiber Communication Systemxe2x80x9d). Below this algorithm will be briefly explained.
The theorem of this algorithm is to assign frequency intervals such that the frequency interval i of any two arbitrary signal beams is different from all other pairs of frequency intervals. This can be understood by modifying Eq. 1 to:
fijkxe2x88x92fi=fjxe2x88x92fkxe2x80x83xe2x80x83(2)
Here, if the number of channels (number of channels) is M, mixe2x88x921 (2xe2x89xa6ixe2x89xa6M) is an integer, and the minimum frequency difference of the four wave mixing light is xcex94f, then the frequency fi of each signal beam is expressed by:
fi=fixe2x88x921mixe2x88x921xc3x97xcex94fxe2x80x83xe2x80x83(3)
In this case, the frequency interval of any two arbitrary channels is the partial sum of the integers m1xcx9cmMxe2x88x921 multiplied by xcex94f, and the problem of finding a frequency interval that is a completely unequally spaced channel allocation comprises finding the Mxe2x88x921 integers whose arbitrary partial sums are all different. Here, partial sum means the sum of mixe2x88x921, which represents the frequency separation of any two arbitrary channels among the Mxe2x88x921 integers mixe2x88x921. For example, this denotes m1, m2, m1+m2, m3+m4+m5+m6, etc.
Moreover, the signal beam along with the minimum frequency difference xcex94f of the four wave mixing and the minimum frequency separation (the minimum value of mixe2x88x921xc3x97xcex94f) of the signal light are determined in consideration of the degree of stability of the oscillating frequency of the semi-conductor laser using as an optical source, the spectral spread of the signal light and the four wave mixing light, the transmission bandwidth of the optical filter, the optical amplifier bandwidth used as a repeater, etc., and xcex94f and mixe2x88x921 are assigned the greatest lower limit.
In connection with the above, presently completely unequally spaced channel allocation has only been obtained up to 12 channels. Therefore, when the unequally spaced channel allocation is expanded, for example, to 16 channels, a method of repeatedly disposing completely unequally spaced channel allocation of 8 channels has been proposed as a alternative means (see J. S. Lee et al., xe2x80x9cPeriodic allocation of a set of unequally spaced channels for upgradable dense-WDM application using dispersion-shifted fibersxe2x80x9d, OFC ""98, FC 5, 1998).
In general, optical fiber amplifiers bandwidth is limited, for example, to 1529xcx9c1560 nm. In addition, the bandwidth is decreased by cascading. In addition, when the difference between the wavelength of the signal light and the zero dispersion wavelength becomes large, waveform degradation due to the chromatic dispersion becomes large.
Because of this, it is preferred that the frequency bandwidth that M signal beams with completely unequal spaced channel allocation occupy (referred to hereinbelow as xe2x80x9coccupied bandwidthxe2x80x9d) be as narrow as possible. In the above-mentioned publication, since the occupied bandwidth of these M signal beams is given as (m1+m2+ . . . mMxe2x88x921)xc3x97xcex94f, a theorem is disclosed wherein the frequency interval of the completely unequally spaced channel allocation is set so that m1+m2+ . . . mMxe2x88x921 is at a minimum.
Here, one example of completely unequally spaced channel allocation for 8 channels which minimizes the occupied bandwidth when the minimum frequency difference xcex94f, between the signal and the four wave mixing light is 50 GHz and the minimum frequency interval (minimum channel allocation) is 3 xcex94f =150 GHz, is shown in Table 1.
As is clear from this Table, in the completely unequally allocated 8 channels, 2.15 (=194.45-192.30) THz is necessary for the occupied bandwidth, and it is understood that this is a great increase compared to the 1.05 (=150 GHzxc3x977) occupied bandwidth in the case of an even allocation by minimum channel allocation.
In addition, the minimal occupied bandwidth necessary for completely unequally spaced channel allocation increases according to the number of channels as shown in FIG. 2, where xcex94f=50 GHz, and the minimum channel allocation of 3xcex94f=150 GHz. Moreover, in the following explanation, the occupied bandwidth is represented by wavelength. That is, the occupied bandwidth of a completely unequally spaced channel allocation for 8 channels is 17.23 (=1558.98-1541.75) nm.
As is clear from this Table, in contrast to the 4.8 nm occupied bandwidth of completely unequally spaced channel allocation for 4 channels, the occupied bandwidth of completely unequally spaced channel allocation for 8 channels is three times as high, and the occupied bandwidth of completely unequally spaced channel allocation for 12 channels is 9 times this. That is, when attempting to increase the number of channels with completely unequally spaced channel allocation, the occupied bandwidth increases drastically, and exceeds the 1529xcx9c1560 nm range, which is the amplification band of the general use optical fiber amplifier.
In addition, even when the unequally spaced channel allocation is expanded to 16 channels by repeatedly allocating the completely unequally spaced channel allocation of 8 channels as in the above-mentioned publication, as is shown in FIG. 1A, the occupied bandwidth becomes greater than 17.23xc3x972 [nm], exceeding the amplification bandwidth (about 30 nm) of generally used optical fiber amplifiers, and there is the problem that channels are produced that cannot be amplified.
Furthermore, when combining a plurality of completely unequally spaced channel allocations as in the above-mentioned publication, the entire frequency will not be completely unequally spaced channel allocation. Therefore, for example, when the completely unequally spaced channel allocation of 4 channels is expanded to 16 channels by combining 4 of them, the occupied bandwidth can be accommodated by the amplification band of the generally used optical fiber amplifier, but the unequally spaced channel allocation becomes incomplete, and there is the problem that it is not possible to completely separate the four wave mixing light from the signal beam.
Thus, the expansion of the number of channels with completely unequally spaced channel allocation is limited due to the increase of this occupied bandwidth. In addition, in the simple combination of a completely unequally spaced channel allocation, the influence of four wave mixing light due to incompletely unequally spaced channel allocation becomes large.
In consideration of the above, it is an object of the present invention to provide a wavelength allocation method, with a transmission equipment and a receiving equipment using this method, and a WDM transmission system wherein the influence of the four wave mixing light can be mitigated even while there is incompletely unequally spaced channel allocation by devising a plurality of completely unequally spaced channel allocation combinations, and make possible both a decrease in the occupied bandwidth and an increase in the number of channels.
The first characteristic of the present invention is that much of the power of the four wave mixing light is generated by the signal beam in the zero dispersion wavelength region of the optical fiber, and that it decreases with increased distance from the zero dispersion wavelength. Based on this, completely unequally spaced channel allocations is combined.
For example, when transmitting on a wavelength multiplexed signal beam in the 1.55 xcexcm band with a 1.55 xcexcm dispersion shifted optical fiber, completely unequally spaced channel allocation of all channels is preferred, but as described above, there are cases in which the occupied bandwidth exceeds the amplification band. Therefore, a plurality of completely unequally spaced channel allocations are combined, and further, the number of channels with completely unequally spaced channel allocation in the wavelength range separated from the zero dispersion wavelength is minimized, and the total occupied band is made narrow. Thereby, it is possible to partition many channels in a limited occupied bandwidth. For example, in the relationship between the number of channels and the occupied bandwidth shown in Table 2, when wavelength multiplexing 16 channels, by combining a completely unequally spaced channel allocation of 8 channels and a completely unequally spaced channel allocation of 4 channels, it is possible to keep the total occupied bandwidth to 30 nm or less.
However, as shown in FIGS. 1A and 1B, the completely unequally spaced channel allocations of 8 channels in the zero dispersion wavelength xcex0 are partitioned, and the completely unequally spaced channel allocation for 4 channels separated from the zero dispersion wavelength are partitioned. In FIG. 1A, by providing a predetermined wavelength allocation between each completely unequally spaced channel allocation, the wavelength allocation for 16 channels is set by one completely unequally spaced channel allocation for 8 channels and two completely unequally spaced channel allocations for 4 channels. In FIG. 1B, by sharing the neighboring channels of each completely unequally spaced channel allocation (the wavelength allocation of the completely unequally spaced channel allocation is 0), wavelength interval for 16 channels is set by one completely unequally spaced channel interval for 8 channels, two completely unequally spaced channel allocation for 4 channels, and part of a completely unequally spaced channel allocation for 4 channels.
The present invention is characterized in that when
N1xe2x89xa7N2xe2x89xa7N3xe2x89xa7 . . .  greater than Njxe2x89xa7Nkxe2x88x921xe2x89xa7Nk,
the N1-channels are selected so as to be completely unequally allocated in the first wavelength region including the zero dispersion wavelength xcex0, and then in the next wavelength regions adjacent to it, N2-channels, N3-channels, etc., which are smaller than N1 are completely unequally allocated, and by their combination, a wavelength interval for N-channels that can carry out wavelength multiplexed transmission is established. That is, the number of channels of the completely unequally spaced channel allocation changes depending on the amount of the dispersion (the difference with the zero dispersion wavelength xcex0).
Thereby, in the region of the zero dispersion wavelength, due to the completely unequally spaced channel allocation of N1-channels, it is possible to completely separate the four wave mixing light and the signal beam. In addition, by the combining of the completely unequally spaced channel allocation of the N2-channels, N3-channels, etc., as a whole there is incompletely unequally spaced channel allocation, but because the wavelengths of these signal beams are separated from the zero dispersion wavelengths, the power of the generated four wave mixing light is small, and it is possible to mitigate the influence of the four wave mixing light because of the incompletely unequally spaced channel allocation. For example, by making the first wavelength region xcex0xc2x110 [nm], the influence of the four wave mixing light can be made small.
The second characteristic of the present invention derives from the fact that the zero dispersion wavelength of an actual transmission line using a dispersion shifted fiber changes in the longitudinal direction, and based on this fact, a plurality of completely unequally spaced channel allocations are combined.
Actual transmission line connects in sequence dispersion shifted fibers having a length of 2 km. For example, an 80 km transmission line comprising one relay interval connects 40 dispersion shifted fibers. Here, an example of a histogram of the zero dispersion wavelength of a 1.55 xcexcm dispersion shifted fiber is shown in FIG. 2. The number of fiber samples is 100.
As shown in the figure, it can be understood that the zero dispersion wavelengths of the 100 dispersion shifted fibers are not uniform, but scattered. That is, the probability that the zero dispersion wavelength exists is comparatively high in the wavelength region around 1500 xcexcm (here, 1545xcx9c1560 nm), and low in the other wavelength regions. Therefore, in the transmission line connecting in sequence this dispersion shifted fiber, it can be understood that the zero dispersion wavelength is scattered (fluctuating) in the longitudinal direction.
The present invention is characterized in that, when
N1xe2x89xa7N2xe2x89xa7N3xe2x89xa7 . . .  greater than Njxe2x89xa7Nkxe2x88x921xe2x89xa7Nk,
as shown in FIG. 3A, first the N1-channels are chosen so as to have completely unequally spaced channel allocation in the wavelength region where the zero dispersion wavelength exists most frequently. In addition, in the wavelength regions in which the frequency is lower than this wavelength region, the N2 channels, N3-channels, etc., which are smaller than the N1-channels are made completely unequally allocated, and these are combined. That is, the number of channels with completely unequally spaced channel allocation changes depending on the probability with which a zero dispersion wavelength exists. Moreover, in the present Specification, the peak where the zero dispersion wavelength most frequently exists, that is, the mode of the zero dispersion wavelength, will be referred to as the xe2x80x9cmode zero-wavelength dispersion wavelength xcexAxe2x80x9d.
In addition, the occupied bandwidth with respect to the number of channels with completely unequally spaced channel allocation has the relationship shown in Table 2, and thus by combining a plurality of completely unequally spaced channel allocations, making the number of channels with completely unequally spaced channel allocation small in the wavelength region where the zero-wavelength dispersion exists least frequently, the occupied bandwidth as a whole becomes narrow. That is, in a limited occupied bandwidth, it is possible to allocate many channels. This is identical to the first characteristic of the present invention.
In addition, when combining the plurality of completely unequally spaced channel allocations as described above, the entire frequency interval is not completely unequally allocated. However, this means that the wavelength region where the zero dispersion wavelength exists least frequently is not continuous with the zero dispersion wavelength seen from the transmission line as a whole (the length of the transmission path having this zero dispersion frequency is relatively short), and the power of the generated four wave mixing light is small. Because of this, by combining the completely unequally spaced channel allocations of N2-channels, N3-channels, . . . , as a whole it becomes incompletely unequally spaced channel allocation, but the power of the generated four wave mixing light becomes small, and it is possible to mitigate the influence of the four wave mixing light due to the incompletely unequally spaced channel allocation.
This situation is shown in FIG. 4. This shows the spectrum when carrying out incompletely unequally spaced channel allocation of 16 channels by combining the completely unequally spaced channel allocation of 8 channels with the completely unequally spaced channel allocation of 4 channels, and transmitting over a 640 km of dispersion shifted fibers. In order to suppose the worst conditions, the zero dispersion wavelengths xcexA (=1552.52 nm) in each span (80 km) are all equal, and at the standard deviation "sgr" (=5 nm) for each 2 km within each relay interval, the zero dispersion wavelength is irregular. The large 16 peaks represent the signal light, and the many small spiked peaks show four wave mixing lights.
From FIG. 4, it can be understood that the signal beam power in the neighborhood of the mode zero dispersion wavelength xcexA is comparatively lessened. The cause of this is that the power of the signal beam itself is lessened because it generates new light by four wave mixing. However, channels separated from the mode zero-wave dispersion, for example channel 1 and channel 16, are not contributing almostly the four wave mixing light generation. That is, it can be understood that the power of the generated four wave mixing light is large in the mode zero dispersion wavelength xcexA neighborhood. In addition, outside the wavelength region 20 nm (xcexAxc2x12"sgr") in the neighborhood of the mode zero dispersion wavelength xcexA, the influence of the four wave mixing is small.
In the example of channel allocation shown in FIG. 3, completely unequally spaced channel allocation of fewer channels is carried out according to the separation from the mode zero dispersion wavelength, and this is identical to FIG. 3A in terms of their being combined. A characteristic of the present example is setting the predetermined wavelength interval xcex94xcexK between each of the completely unequally spaced channel allocations. This wavelength interval xcex94xcexK is determined by the relationship between the frequency grid defined by ITU-T and the unequally allocated wavelengths, the amplification band, etc. Moreover, FIG. 3A can be said to be the case wherein xcex94xcexK=0. In addition, in FIG. 3A, the upper limit of the number of partitioned channels is N1+N2+N3+N4+N5xe2x88x924. In FIG. 3B, the upper limit of the number of partitioned channels is N1+N2+N3+N4+N5.
In this manner, in the present invention even when there is incompletely unequal channel allocation as a whole in combining a plurality of completely unequally spaced channel allocations, by changing the number of channels of completely unequally spaced channel allocation according to the amount of dispersion and how frequently the zero dispersion wavelength exists, it is possible to implement both a decreased occupied bandwidth and an increase in the number of channels while mitigating the influence of four wave mixing light.
In addition, by providing a transmission equipment having several means for realizing the above wavelength allocation method and a receiving means for receiving the wavelength multiplexed light transmitted from this transmission equipment, it is possible to construct a wavelength multiplexed transmission system having many channels effectively using the optical amplifier bandwidth.
As explained above, in the present invention, even when there is incompletely unequal channel allocation as a whole in combining a plurality of completely unequally spaced channel allocations, by changing the number of channels of completely unequally spaced channel allocation depending on the amount of chromatic dispersion of the optical fiber and how frequently the zero dispersion wavelength exists, it is possible to implement both a decrease the occupied bandwidth and an increase in the number of channels while mitigating the influence of four wave mixing light. In this manner, it is possible to construct a wavelength multiplexed transmission system with many channels effectively using the optical wavelength band.